1. Field of the Invention
This invention relates to a voltage detecting device, such as an electrical circuit, for detecting the voltage of a predetermined part of an object under measurement, and more particularly to a voltage detecting device which operates upon the principle that the refractive index of an electro-optical material depends on the voltage of a predetermined part of an object under measurement.
2. Description of the Related Art
A variety of voltage detecting devices comprising electrical circuits have been used to detect the voltage of a predetermined part of an object. One example of a voltage detecting device employs a probe which is brought into contact with the predetermined part of the object to detect the voltage in the part of the object. In another example of a voltage detecting device, a probe is held away from the predetermined part of the object and an electron beam is applied to the predetermined part of the object to detect the voltage.
In the art of voltage detecting devices, there is a high demand for a method of detecting the voltage of a minute part of an object, such as a small integrated circuit, with high accuracy and without affecting the conditions of the minute part. A voltage detecting device of the type with a probe which is brought into contact with a predetermined part of an object under measurement has many disadvantages. It is difficult to bring the probe into contact with a small part of an integrated circuit. Even if it was possible to bring the probe into contact with a part of the integrated circuit, it is difficult to analyze the operation of the integrated circuit correctly based only upon the voltage data. Furthermore, the operating conditions of the integrated circuit are changed by bringing the probe into contact with the integrated circuit.
The voltage detecting device of the type employing an electron beam while a probe is held away from an object under measurement in order to detect a voltage also has many problems. The part to be measured must be exposed in a vacuum and may be damaged by the electron beam.
In the conventional voltage detecting devices, the operating speed of the detector cannot keep up with a high-speed change of voltage. Therefore, conventional voltage detecting devices have the disadvantage that voltages that rapidly change, in integrated circuits for instance, cannot be accurately detected.
In order to solve the above problems, a voltage detecting device operating upon the principle that the polarization of a light beam is changed by the voltage of a predetermined part of an object under measurement has been disclosed in unpublished Japanese patent application number 137317/87 filed in Japan on May 30, 1987. FIG. 7 is a diagram showing the arrangement of the Applicants' previously disclosed voltage detecting device.
As shown in FIG. 7, the voltage detecting device comprises an optical probe 52, a light source 53 including for instance a laser diode, an optical fiber 51 for guiding a light beam from the light source 53 through a condenser lens 60 to the optical probe 52, and an optical fiber 92 for leading a reference light beam from the optical probe 52 through a collimator 90 to a photo-electric conversion element 55. The device further comprises an optical fiber 93 for applying an emergent light beam from the optical probe 52 through a collimator 91 to a photo-electric conversion element 58, and a comparison circuit 61 in which the output electrical signals of the photo-electric conversion elements 55 and 58 are subjected to comparison.
Electro-optical material 62, such as an optically uniaxial crystal of lithium tantalate (LiTaO.sub.3), is enclosed in the optical probe 52. The end portion 63 of the optical probe 52 is in the form of a circular truncated cone. A conductive electrode 64 is formed on the cylindrical wall of the optical probe 52. A reflecting mirror 65 made of a metal film or dielectric multilayer film is provided on the end face of the end portion 63 of the optical probe 52.
Provided inside the optical probe 52 are a collimator 94, condenser lenses 95 and 96, a polarizer 54 for extracting only a light beam having a predetermined polarization component out of a light beam outputted by the collimator 94, and a beam splitter 56 for dividing the light beam having the predetermined polarization component, which is provided by the polarizer 54, into the reference light beam and an incident light beam, and applying an emergent light beam from the electro-optical material 62 to an analyzer 57. The reference light beam and the emergent light beam are applied through the condenser lenses 95 and 96 to the optical fibers 92 and 93, respectively.
In a voltage detection device 50, as described above, the conductive electrode 64 formed on the cylindrical wall of the optical probe 52 is grounded. The end portion 63 of the optical probe 52 is set close to an object under measurement, such as an integrated circuit (not shown). As a result, the refractive index of the end portion 63 of the electro-optical material 62 in the optical probe 52 is changed. more specifically, in an optically uniaxial crystal, the difference between the refractive index of an ordinary light beam and that of an extraordinary light beam in the plane perpendicular to the direction of popagation is changed.
The output light beam of the light source 53 is applied through the condenser lens 60, the optical fiber 51, and the collimator 94 in the optical probe 52 to the polarizer 54. The polarizer 54 outputs a light beam having the predetermined polarization component of intensity I. The output light beam of the polarizer 54 is applied through the beam splitter 56 to the electro-optical material 62 in the optical probe 52. The reference light beam and the incident light beam formed by the beam splitter 56 are of intensity I/2. As was described above, the refractive index of the electro-optical material 62 is changed by the voltage of an object under measurement. Therefore, the incident light beam applied to the electro-optical material 62 is changed in polarization at the end portion 63 depending on the change of the refractive index and is then reflected by the reflecting mirror 65. The reflected light is applied, as the emergent light beam from the electro-optical material 62, to the beam splitter 56. The polarization of the incident light beam is changed in proportion to the difference in refractive index between an ordinary light beam and an extraordinary light beam which attributed to voltage, and in proportion to a value 2 l where l is the length of the end portion 63 of the electro-optical material 62.
The emergent light beam is applied to the analyzer 57 by the beam splitter 56. The intensity of the emergent light beam applied to the analyzer 57 is reduced to I/4 by the beam splitter 56. If the analyzer 57 is designed to transmit only a light beam having a polarization component perpendicular to the polarization component of the polarizer 54, then the analyzer changes the intensity I/4 of the emergent light beam applied to it to (I/4) sin.sup.2 (.pi./2).multidot.V/V.sub.O) where V is the voltage of the object under measurement, and V.sub.O is the half-wavelength voltage. The emergent light beam is then applied to the photo-electric conversion element 58.
In the comparison circuit 61, the intensity I/2 of the reference light beam subjected to photo-electric conversion by the photo-electric conversion element 55 is compared with the intensity (I/4) sin.sup.2 ((.pi./2).multidot.V/V.sub.O) of the emergent light beam subjected to photo-electric conversion by the photo-electric conversion element 58.
The intensity (I/4) sin.sup.2 ((.pi./2).multidot.V/V.sub.O) of the emergent light beam depends on the change in refractive index of the end portion 63. of the electro-optical material 62 which is due to a variation in voltage. Therefore, the voltage at a predetermined part of an object under measurement, such as an integrated circuit, can be detected from the intensity.
As was described above, the voltage detecting device of FIG. 7 is designed so that the voltage of a predetermined part of an object under measurement is detected from the change in refractive index of the end portion 63 of the electro-optical material 62 which is caused when the end portion 63 of the optical probe 52 approaches the object. Therefore, with the device of FIG. 7 the voltage of a minute part of an integrated circuit which is difficult for a probe to contact can be detected with the optical probe 52 held away from the minute part. Further, the voltage of a minute part of an integrated circuit which is affected when the integrated circuit is contacted with a probe can be detected with the optical probe 52 held away from the minute part. Furthermore, a pulse light source such as a laser diode outputting an optical pulse which is extremely short in pulse width may be employed to sample the rapid voltage changes of the object at considerably short time intervals. Alternatively, a d-c light source and a high-speed response detector such as a streak camera may be used to measure the quick voltage changes of the object with high resolution. Thus, the rapid voltage changes can be detected with high accuracy using the device of FIG. 7.
The voltage detecting device 50 of FIG. 7, however, has many difficulties. The FIG. 7 device detects the voltage of a predetermined part of an object under measurement from the variation in polarization of the light beam in the electro-optical material 62. Therefore, it is necessary to extract only the light beam having the predetermined polarization component from the output light beam of the light source 53 with the aid of the polarizer 54. It is further necessary to extract the predetermined linear polarization component from the emergent light beam from the electro-optical material 62 with the aid of the analyzer 57. Accordingly, the device has low utilization of light. Furthermore, the device includes the beam splitter 56. Therefore, the intensity of the emergent light beam applied to the analyzer 57 is lower than that of the light beam output from the light source 53. As a result, the voltage detection is of limited accuracy and efficiency. Moreover, the device has a relatively large number of optical components including the polarizer 54, the analyzer 57 and the beam splitter 56. Therefore, accuracy of the optical system is limited. In addition, by detecting the voltage of a predetermined part of an object under measurement from the change in polarization, only the absolute value of voltage can be detected. It is impossible to determine the polarity of the voltage, i.e., it is impossible to determine whether the voltage is positive or negative.